Shocking ! my computer sound sucks !! How to improve it ??

Hello everyone, this is my fist post on Head-Fi but I am visiting the forum for some month already... Ok so I am basically an "ear virgin", I like good sound, good quality and I thought I built a cheap but nice HTPC... But in fact, it sucks!!
Ok let me explain my system. I have a HTPC connected to my yamaha receiver through optical connexion. the sound card is a cheap Terratec Aureon 5.1 PCI, and until yesterday I was quite happy with it, playing my DTS movies... One of my favourite DVD is Eagles: Hell Freezes Over in DTS. I used to play it with my dvd player connected to my receiver through Coax, amazing sound, pure happiness ! yesterday I played it on my HTPC connected to my beamer and I was shocked, the sound was crap, some notes were missing compared to my dvd player, the bass was too strong and the treeble too weak, awful sound
So my question is: do I have a sound card problem or something else?
I thought the DTS signal was just passing through the card, without modifying it. What sound card should I buy to enjoy a good sound in the 100 euros range (130$)?

Sorry for the bad english and the bad writing...I'll try to improve...

I'm very sorry I wasn't clear enough, so today I will try to be more specific. My computer is connected to my receiver via an optical connexion to get the dolby digital or DTS signal. That "works" more or less since my receiver is detecting the correct signal always. And my computer is connecter to my projector via HDMI. Then my DVD player is connected to my receiver via Coax cable and I get also DD or DTS signal, no problem. I have never encountred any problem reading a dvd with my PC, powerdvd reads it correctly and is configured to send the DD and DTS signal through SPDIF. my problem is that I was never comparing a DVD on both players, the computer and the DVD player, and I just found out that the computer sucks awfully. My Eagle DVD was the revealator as I could'nt hear some guitar parts and the bass was way too strong when played with the PC.
So my question is: Is it a sound card problem because it is cheap and the quality not very good (in that case I buy a new one and I enjoy my sound again) or is it something else, like powerdvd is bad in decoding correctly the DTS sound or whatever. I don't want to spend 100 euros in a new sound card to have again a bad sound quality.
I assume it is a sound card issue, but I thought that the sound card was not involved in the process of sending a DTS signal to the receiver (but I have a very limited knowledge in that topic) is it correct ?
Then my question would be which sound card? I know only the "famous" one in the computer world, not from the HiFi world. I was thinking of the Asus Xonar DX or an auzentech 7.1 cinema.
basically I am lost...

I'm very sorry I wasn't clear enough, so today I will try to be more specific. My computer is connected to my receiver via an optical connexion to get the dolby digital or DTS signal. That "works" more or less since my receiver is detecting the correct signal always. And my computer is connecter to my projector via HDMI. Then my DVD player is connected to my receiver via Coax cable and I get also DD or DTS signal, no problem. I have never encountred any problem reading a dvd with my PC, powerdvd reads it correctly and is configured to send the DD and DTS signal through SPDIF. my problem is that I was never comparing a DVD on both players, the computer and the DVD player, and I just found out that the computer sucks awfully. My Eagle DVD was the revealator as I could'nt hear some guitar parts and the bass was way too strong when played with the PC.
So my question is: Is it a sound card problem because it is cheap and the quality not very good (in that case I buy a new one and I enjoy my sound again) or is it something else, like powerdvd is bad in decoding correctly the DTS sound or whatever. I don't want to spend 100 euros in a new sound card to have again a bad sound quality.
I assume it is a sound card issue, but I thought that the sound card was not involved in the process of sending a DTS signal to the receiver (but I have a very limited knowledge in that topic) is it correct ?
Then my question would be which sound card? I know only the "famous" one in the computer world, not from the HiFi world. I was thinking of the Asus Xonar DX or an auzentech 7.1 cinema.
basically I am lost...

thanks for your replies anyway...

I did a short trawl on the Internet. It seems that PowerDVD can be a bit flaky with DTS with some sound cards. I only have a very old PowerDVD and no DTS receiver so I cannot experiment myself.

Can you try with a different media player such as Nero or WMP ? Are you using a multi-speaker i.e 5.1 set-up, the most common complaint is that PowerDVD with some cards only outputs stereo or hiss, sorry I cannot be more help, I would examine the software before investing money in new hardware.

At the very least get the latest patches for PowerDVD and latest sound card drivers.